In the past many attempts have been made to achieve a truly reliable, interference-immune, compact and effective motion detector for use in protecting vehicles or other structures from unauthorized entry and theft. Many systems have been devised which are sensitive to movement of a vehicle or a person in the region of the protected structure. These, however, have usually exhibited high susceptibility to false alarms due to normal stimuli such as passing traffic, thunder, electrical fields and other natural causes too numerous to mention. Other motion detectors do not respond to shock input in addition to being susceptable to false alarms.
In vehicle alarm systems, attempts have been made to devise a system which responds to stimuli at two different frequencies. An example of such a system is disclosed in U.S. Pat. No. 4,496,809 to J. H. Faust et al. Such a system employs cantilevered springs. Other forms of dual response motion detectors are typified by the system of U.S. Pat. No. 3,559,050 to J. F. Mifsud which employs a pair of separate windings on a single coil form and coupled through an electrical circuit for amplification and phase adjustment. The net result is operation as an accelerometer and as low frequency geophone. Other accelerometers employing a vibrating beam are illustrated in U.S. Pat. No. 4,628,735 to Gordon S. Kirkpatrick which includes a dual beam force sensing element.
Piezoelectric sensors are commonly employed in accelerometers. They exhibit stability however, being of ceramic construction, are often brittle and subject to shock damage.
Hall-effect devices such as those described in SOLID STATE ELECTRONIC DEVICES, B. G. Streetman, Copyright 1980, Prentice-Hall, Inc., Englewood Cliffs, N.J. 07632, at pages 88-90, exhibit a sensitivity to changes in ambient magnetic field. This property makes them useful in applications where a controllable magnetic field may be produced in order to provide an electrical signal from a solid state device. The use of Hall-effect devices in motion detectors to detect dual frequency stimuli has not been heretofore achieved in a useful device.